1. Field of the Invention
This invention relates to electrical brush holders whose function is: (i) to maintain the running surface of any given brush to which it is releasably fastened in a steady, predetermined position during relative tangential motion between the brush and its substrate (i.e., commonly a slip ring or commutator), (ii) to apply a predetermined, approximately constant (compare the data in Table III) mechanical pressure between the brush running surface and the substrate while the brush may wear, and (iii) to conduct electrical current to or from the brush.
The electrical brushes at issue include all conventional “monolithic” brushes (i.e. made in one piece of graphite or graphite-metal mixtures), but are principally metal fiber brushes disclosed in U.S. Pat. Nos. 4,358,699 and 4,415,635, and in the co-pending international patent application Ser. No. 09/147,100 and foil brushes as described in the publication “Production and Performance of Metal Foil Brushes,” P. B. Haney, D. Kuhlmann-Wilsdorf and H. G. F. Wilsdorf, WEAR, 73 (1981), pp. 261-282. The present invention is particularly useful for electrical metal fiber brushes in motors and generators when operating at high current densities, especially in homopolar motors/generators. The present invention includes the use of various technologies referenced and described in the above-noted U.S. Patents and Applications, as well as described in the references identified in the appended LIST OF REFERENCES and cross-referenced throughout the specification by reference to the corresponding number, in brackets, of the respective references listed in the LIST OF REFERENCES, the entire contents of which, including the related patents and applications listed above and the references listed in the LIST OF REFERENCES, are incorporated herein by reference.
2. Discussion of the Background
Sliding electrical contacts, i.e., “brushes,” conduct electrical current between solids, very preponderantly metals, in relative motion. Brushes are in widespread use in various types of electric motors and generators and are also widely used in less common but numerous special applications, e.g. telemetry devices and rotating antennae. Even while to date the traditional “monolithic” (i.e., in the form of a solid piece) graphite-based (i.e., including compacted graphite or various metal-graphite mixtures) brushes are overwhelmingly frequent, they have a number of technological limitations. Specifically, monolithic graphite-based brushes cannot be reliably used over extended periods of time at current densities above about 30 Amp/Cm2, nor at sliding speeds above about 25 m/sec. Further, as a coarse estimate, they waste about one watt per ampere conducted across the brush-substrate interface (i.e. the equivalent of one Volt) in terms of Joule and friction heat together. Further, monolithic brushes emit significant intensities of electromagnetic waves (i.e., they are electrically very noisy so as to interfere with radio and similar signal reception), and finally they wear into a powdery debris that can be highly detrimental in electrical machinery, especially aboard submarines.
As a result of these shortcomings of traditional monolithic brushes, a number of otherwise very attractive technological developments are stymied for lack of electrical brushes which will conduct reliably over extended time periods, much higher current densities at low losses up to much higher speeds. Most importantly impacted are so-called “homopolar” motors and generators. They have potentially very high power densities and would be excellent for Navy as well as commercial ship drives, among others, but typically require current densities in excess of one hundred Amperes per cm2 to be conducted across interfaces of metal parts relatively moving at sustained speeds up to 30 m/sec or even more while producing or requiring EMF's of only 20V or so. The requirements of homopolar machinery in terms of current densities and speeds can thus not be fulfilled by monolithic brushes, and in any event a loss of 2 Volts per monolithic brush pair, i.e., in and out, is prohibitive for homopolar machines.
In previous inventions, particularly in the Patent Application “Continuous Metal Fiber Brushes, [1]” the capabilities of metal fiber brushes, including multitudes of essentially parallel hair-fine metal fibers, are outlined. Metal fiber brushes are intrinsically capable of easily conducting the desired current densities and to do so up to at least 70 m/sec with a total loss in the order of 0.1 Volt per brush. At the same time such brushes are electrically very quiet. These superior qualities derive from large numbers of separate electric “contact spots,” namely at the fiber ends at the brush “working surface” sliding along the brush-substrate interface, through which the current is physically conducted on a microscopic scale. That the current is conducted across solid interfaces only through a restricted number of contact spots, whose total area amounts to only fractions of one percent of the macroscopic area of contact, is a well-known general physical phenomenon. To a large extent the poor qualities of monolithic brushes arise from their small number of contact spots, namely in the order of ten per brush. As a result, the current flow lines in monolithic brushes are not rather uniformly distributed, as they are in metal fiber brushes, but they are “constricted” [2] at the few contact spots. This causes the corresponding “constriction resistance” that represents in the order of one third the resistance of monolithic brushes. This constriction resistance is eliminated in metal fiber brushes on account of their large number of contact spots.
The superiority of metal fiber brushes does not only derive from their thousands of evenly distributed contact spots, but also because at their contact spots, bare metal meets bare metal, ideally separated only by a double monomolecular layer of adsorbed water. Fortuitously, this most favorable type of lubrication, which prevents cold-welding and accommodates the relative motion between brush and substrate at a “film resistivity” of only σF≅1×10−12 Ωm2 and average friction coefficient (μ) of about 0.3, establishes itself automatically at any modest ambient humidity, provided that the area of any one brush is not too large and there are gaps between the brushes so as to permit access of the moisture to the substrate and that undue contamination with oils, etc., is avoided. By contrast, monolithic brushes deposit a lubricating graphitic layer through which the current must flow at much higher electrical film resistivity and which typically is also overlaid by the already indicated film of adsorbed moisture [3]. Further, the body resistance of graphitic brushes can be significant while it is always negligible for metal fiber brushes. Finally, monolithic brushes are hard and “bounce.” At increasing speeds, the “brush bounce” must be counteracted by an increasingly strong pressure between brush and substrate at the correspondingly increased friction power loss. This syndrome limits the sliding speed of monolithic brushes to about 25 m/sec, as already indicated, whereas metal fiber brushes are intrinsically flexible (i.e., have a much larger “mechanical compliance”). Therefore, metal fiber brushes can and should be mechanically lightly loaded and can be operated to high speeds with minor friction heat loss.
Metal foil brushes closely resemble metal fiber brushes except they are composed not of substantially parallel fibers but of thin parallel foils [4]. Consequently, metal foil brushes typically have many fewer, but otherwise the same kind of, contact spots. Thus, metal foil brushes are very similar to metal fiber brushes but cannot match their attainable current densities, sliding speeds and low power losses. At any rate, foil brushes are based on the same principle as metal fiber brushes, namely, electrical contact to the substrate at a large number of microscopically small, bare metal-metal contact spots, optimally lubricated by a double monomolecular layer of adsorbed water. Hence, in terms of the number of contact spots per unit working surface area (i.e., “contact spot density”), and mechanical load per contact spot, the same theory applies to metal foil as to metal fiber brushes [4].
As stressed, on account of their different geometry, foil brushes include a substantially smaller density of contact spots than well-constructed metal fiber brushes. By numerical example, the working surface of a typical metal fiber brush constructed of d=50 μm copper wires of about f=15% packing fraction contains roughly 10,000 contact spots per cm2, namely, one at each of the individually flexible fiber ends. In a foil brush with df=25 μm thick parallel foils and f=50% packing fraction, there are about 600 contact spots per cm2, located at the foil edges sliding on the substrate, with an estimated three contact spots per foil edge. Correspondingly, without suitable modifications of the substrate, foil brushes will be very superior to monolithic brushes, but fall short of metal fiber brushes [4].
In typical use, both types of brushes are expected to wear by similar length changes in the course of their life times, e.g. several millimeters (¼″) or up to an inch, during which time the mechanical brush force should be kept roughly constant. The major differences between monolithic and metal fiber brushes include:                lower mechanical pressure, namely several pounds per square inch for monolithic brushes, versus about 1 Newton per square centimeter≅1 pound per square inch for fiber brushes.        higher current densities, i.e., up to 30 Amp/cm2 200 Amp/in2 for monolithic brushes and up to 300 Amp/cm2 2000 Amp/in2 for fiber brushes,        at the indicated maximum tolerated current densities and speeds up to 70 m/sec, total losses of below 0.3V per ampere conducted, including friction and Joule heat, for fiber brushes and about 1 V/ampere conducted for monolithic brushes.Correspondingly, the mechanical stiffness as well as the electrical resistance of, and hence the electrical loss in, the current leads to or from the brushes, are always inconsequential for monolithic brushes but become very important for metal fiber brushes when used anywhere near their current carrying capability.        
As a result, the mechanical force can be applied to monolithic brushes via springs or any other desired mechanical means, while the current is led to or from the brushes either through the same springs and/or through ordinary flexible electrical cabling connected in parallel with the brush force applicator. However, this is not a viable option for demanding applications of metal fiber and foil brushes because 1) the weaker springs needed for them will unavoidably have an electrical resistance comparable to or higher than that of the brushes, unless they were to be cooled to cryogenic temperatures and even perhaps be made of a superconducting material, and 2) the incidental forces exerted on the brush by flexible cables with adequately low electrical resistance above cryogenic temperatures will rival or exceed the applied spring force.
The problem to be solved for metal fiber brushes used at high current densities above cryogenic temperatures is therefore how to apply a controllable light brush pressure and at the same time to establish a low resistance electric contact to or from the brushes. A system with these characteristics would in fact be applicable to any electrical brush, whether of metal fiber or monolithic type, under any running conditions, but it would be definitely necessary only for the indicated high-current-density use of metal fiber and foil brushes.